High Pressure in Fuel Tank

[I.T.]

I have an Audi A6 2.8 Avant 1998, U.K.
My problem is when the car is running, the fuel pressure in the tank gets so high you can hear air hissing out of the filler cap area. I can fill up with fuel as normal and apart from the fuel level indicator being a little erratic the car is running OK and has been with this problem for over a week.
Any thoughts? Fuel pressure regulator? Fuel Cap?

[Peter D]

There is a vent pipe form the tank to the Charcoal filfer in the drivers dise in wheel arch. Next to the filter is equipped with a tank protector valve. This may have failed. Are you sure the tank is pressurising not pulling a vacuum. Sicl you nose close to the filler when you open it. If it is a vacuum you will not smell fuel whilst it is hissing.
I think the problem is the Gravity valve

Check valve for through-flow
valve vertical: open,
valve tilted 45 °: closed

This is in the filler pipe and is designed to let air in as fuel is being used up.

Regards Peter

[I.T.]

Thanks for the reply, yes it does appear to be pulling a vacuum. So I should check the Gravity Valve?

[Peter D]

If you need a drawing send me your email address. Regards Peter

[Crasher]

The tank vent valve solenoid in the engine bay is stuck open. I have seen this make A4 tanks collapse to the extent that the fuel pump is drawn up to the top of the tank and the electrical connectors short out on the level sender, blowing the fuse and probably causing sparks in the tank. Clamp the hose from the valve and see if the problem stops, it should also show up on a fault code read as a mixture adaptation too rich error and

01247 Activated charcoal filter solenoid valve 1 -N80

[I.T.]

Thanks for the replies, just to update forum:
replacing the following parts fixed the problem
4B0 201 803B. CARBON ACTIVATED CHARCOAL CANNISTER.
077 133 517 C. EVAP PURGE VALVE.